Extracorporeal kidney stone disintegrators, known as lithotripters, are well known in the art. A truncated ellipsoidal reflector is filled with water, and is coupled to the body of a patient suffering from kidney stones. A spark gap mechanism is located at one focus point of the ellipsoid within the truncated reflector, and the reflector is positioned adjacent the patient's body so that the second focus point of the ellipsoid lies on the kidney stone to be disintegrated. The reflector is filled with water and the open end of the truncated ellipsoid is closed with a diaphragm which is pressed against the patient's body. A spark is generated across the gap at the focus point of the truncated ellipsoid, and this causes some of the water in the immediate vicinity to be flashed into steam, and a shock wave is generated. The energy of the shock wave is focused on the second focus point of the ellipsoid, and since this lies on the kidney stone, the kidney stone is subjected to rather considerable shock energy, and is fragmented. In a period of time usually on the order of an hour the kidney stone, subjected to repeated shock waves, is fragmented into small particles that pass out with the urine, thereby obviating the necessity of surgery.
The reflector is conventionally made of metal, brass being one satisfactory example. Gas bubbles form in the water in the reflector as a result of steam generation by the spark jumping the gap, by some dissociation of the hydrogen and oxygen constituting the water, and by release of air dissolved in the water. The open end of the reflector is oriented upwardly, and the gas released tends to accumulate beneath the diaphragm where it interferes with energy transfer.
One effective structure for degasification of water in lithotripter is disclosed and claimed in our prior U.S. Pat. No. 4,715,375. This includes a very thin, flexible resilient sack, bag or pouch somewhat in the nature of an upright balloon which encloses the spark gap structure. Gas is retained within this balloon and is readily removed therefrom as discussed in the aforesaid U.S. patent.
We have discovered that occasionally a spark instead of jumping the spark gap structure will jump from the spark gap structure to the metallic reflector. This results in puncturing the balloon, and this in turn requires interrupting the kidney stone disintegration to replace the balloon.